The invention relates to a line scanning or horizontal deflection circuit for a television receiver, comprising thyristors.
It constitutes an improvement to the circuits described for example in U.S. Pat. Nos. 3,449,623 and 3,906,306.
The basic horizontal scanning circuit comprises essentially: a source, a first switching device including the inductance of the deflector coil mounted in series with a trace capacitor of high capacitance, and a first thyristor-diode pair operating as a scanning switch, and a second switching device including a reactive circuit constituted by an inductance and a capacitive network, and a second thyristor-diode pair causing an oscillating current to pass into the scanning switch, so as to cancel out the current in the first thyristor while ensuring at the same time that the energy is transferred from the source to the scanning coil associated with trace capacitor.
The gate of the second thyristor is coupled to the line or horizontal oscillator of the receiver.
The invention concerns the manner of elaborating the gate signal of the first thyristor operating as a scanning switch.
Several solutions exist at the present moment.
The gate is driven directly by an active circuit, such as the line oscillator (horizontal). In a first solution using a capacitive coupling, it is necessary to have a DC voltage supply of high value (25 volts). In a second solution, in direct connection, a heavily negative DC supply voltage (-25 volts) becomes necessary. At start up it is difficult to rapidly obtain these voltages.
Another solution consists in using the signal across the further inductance connecting the second switching device to the DC supply source. This signal which becomes positive previously to the conduction of the first thyristor becomes negative during the recovery time thereof and the horizontal flyback or retrace periods. The tapping of this signal requires an auxiliary winding coupled to this further inductance connecting the scanning circuit to the supply source. In a variation of this solution an active regulation is used in series with said inductance. But in this case, the voltage available for driving the gate falls to zero during the opening period of the active regulation element, and the operation becomes uncertain.
A third solution consists in using a damping and protection network, formed by a resistor and a capacitor in series, mounted in parallel with the second thyristor-diode pair, this network being intended for protecting the thyristor by reducing the voltage gradient which is applied thereto. The voltage existing at the terminals of the capacitor is then taken to drive the gate of the first thyristor. But then the current required for its opening passes through the resistor mounted in series with the capacitor and, for this reason, the power dissipated therein becomes prohibitive.
It can be seen that all these solutions present different disadvantages: dissipation of energy, sensitivity to the regulation and uncertainty of operation at start up, increase of the number of components.